Silver Bullet Science?

Kindra Gordon

Fri, 2001-06-01 12:00

Gene markers will add precision, but they won't eliminate EPDs or management. Finding a beef cow's perfect mate may be getting more precise. That's thanks in part to gene marker technology that can pinpoint which beef sires will produce desirable end-products. GeneSTAR the first commercial DNA test for marbling is set to be released in the U.S. this summer. And, other markers for carcass traits are

Gene markers will add precision, but they won't eliminate EPDs or management.

Finding a beef cow's perfect mate may be getting more precise. That's thanks in part to gene marker technology that can pinpoint which beef sires will produce desirable end-products.

GeneSTAR™ — the first commercial DNA test for marbling — is set to be released in the U.S. this summer. And, other markers for carcass traits are being investigated by researchers at universities and private companies around the world.

In the future, such DNA technology could also help producers pre-select genetics for reproductive traits, feed efficiency and even disease resistance. The door to these opportunities swings farther open each day as researchers learn more about the DNA sequences — or gene markers — for specific traits.

Here's how markers work: Even though the gene for a specific trait like tenderness has not yet been identified, markers related to the tenderness gene have been found. Since these markers are inherited with the gene from one generation to the next, researchers must only identify the marker and then determine if the allele identified with that marker (or markers) is expressed positively or negatively for a trait.

In short, if a bull is homozygous for the positive allele, then all offspring will receive the beneficial gene for that trait. If a bull is heterozygous for the gene influencing the trait, half of the progeny will receive the beneficial allele for the trait, and half will have the negative allele. The DNA test then becomes a tool for distinguishing these offspring for the specific gene in question.

Think of the possibilities: Feeders can DNA-test incoming calves for their genetic potential for marbling, tenderness and other traits. They can then feed them to match their predicted performance and market them to specific consumer markets. It has the potential to eliminate the genetics that produce a less than stellar steak. But, it's not a silver bullet.

Multi-Trait, Management Important

The advent of this high-tech science won't oust the importance of expected progeny differences (EPDs) or management in determining animal performance, says John Pollak. He's a professor of animal breeding and genetics at Cornell University.

In regard to gene marker information, Pollak says, “It becomes another source of information for that animal. Now, we need to combine that information with the rest of the data we may have on him.”

Pollak issues that advice for two reasons. First, in most cases the DNA test will be for markers, not the actual gene or mutation causing the difference in progeny performance. While markers do give us more information about expected animal performance, they are only predictors of the gene being tested and don't give all of the answers.

For most traits of interest, many genes influence performance, Pollak says. The value of those genes — for which no DNA test is available — still needs to be included in the selection equation, he adds.

Thus, like EPDs, genetic markers can only guide the conventional breeding process. That won't change until researchers have sequenced — and learn more about — the entire cattle genome.

Therefore, even though DNA tests like GeneStar will give producers the ability to select for a specific gene — in this case marbling — single trait selection can be a gamble.

“If we select for just one trait, we have to be careful. Correlated traits also change in response to the selection for a single trait and not always in the direction we desire. Therefore, it's important to know more about the effects of that gene and others coming through the pipeline,” Pollak says.

Add to that uncertainty the mounting evidence that management plays a role in how genetics are played out.

“Even though an animal may have a positive allele of a gene for a desired trait, there is no guarantee,” Pollak says.

Thus far, data collected on 11,000 carcasses in the checkoff-funded Carcass Merit Project are indicating management has a very significant influence on shear force values for tenderness, he says.

The project is evaluating carcass data from progeny of certain sires to predict each sire's genetic contribution to tenderness. The study is also being used to validate 11 DNA markers. (See “Expect More Information On Sires.”)

Management from birth to slaughter can make a difference in tenderness, Pollak says of the findings. “There is some interesting variation between contemporary groups that we have not yet investigated.”

Ohio State University animal scientist Francis Fluharty echoes that caution about markers and management. He is among a team of Ohio researchers continuing to study markers for tenderness and marbling that they identified last year.

“There's not a magical tenderness marker,” Fluharty says. “Tenderness is something we measure with shear force, and there are several biological and management factors that influence tenderness.

“Markers will only help us identify the potential outcome of an animal. Management strategies can alter that outcome,” Fluharty says.

He gives the example of a feedlot steer with known markers for marbling. “All it means is that the animal has the potential to marble. From ranch-to-rail data previously collected, we know that if that animal gets sick and is treated more than once, he probably won't marble. If he goes on feed at 14-16 months of age and hasn't had proper nutrition up to that point, he probably won't marble.”

Fluharty says number of days on feed and the role a high-carbohydrate diet play in producing intramuscular fat (marbling) are critical management tools that influence performance — no matter what the animal's genetics.

Another hurdle: Markers will identify genes that have heritabilities, too, Fluharty says. “So producers are going to have to be savvy — just as they are with EPDs and accuracies. The decisions are only going to get more complex.”

Because of these variables, he advises, “Markers are just another piece of information that can be used to manage cattle to an optimum endpoint.”

Combining Data For One Number

Despite these cautions, genetic markers will move beef sire selection forward. But, Pollak suggests the wisest use of gene markers is to integrate the information into a genetic evaluation system that weighs all the information about an animal. He says that equates to EPDs for traits that factor in genetic information.

Four universities — Cornell, Iowa State, Colorado State and the University of Georgia — are collaborating to make that a reality.

Through funding from Congress, these four institutions, selected because of their EPD work for breeds in the past decade, formed the National Beef Cattle Evaluation Consortium this May. One of their first projects will be to transfer technology for DNA testing into EPD calculations.

How will it work? As an example, Pollak points to the American Simmental Association that already has a tenderness EPD calculated from progeny performance collected in the Carcass Merit Project.

“The goal is to add DNA information to that equation to get one number that takes into account both sets of data and ultimately gives a better estimate of animal performance,” Pollak says.

“In the end, what we'd like is for a producer to look at an EPD and know genotype information is already included in the assessment. That way producers don't have to factor that information in themselves.”

EPDs for carcass traits with DNA information factored in could be available in a year. Pollak says after this start-up year, the consortium then hopes to expand their scope into areas of health and reproduction.

Looking Into The Future

When it finally arrives, this new science will take some of the guesswork out of future calf crops. And, Fluharty predicts it will have a big impact on marketing cattle.

“Knowing genetic potential will help feed cattle to their projected marbling ability, and so they can meet grid specs,” he says. At the packing plant, DNA tests could be used to cull genetically tough carcasses, he adds.

“This is something alliances — or anyone with an interest in the end consumer — should look at,” Fluharty says. But at the producer level, he says cost could determine where DNA tests are used.

“If the cost comes down to $5 to test an animal to find out whether or not he'll grade Choice, I think you could use it to manage animals and recoup that cost three- to ten-fold in a market that values the end-product — depending on the cost of feed and the Choice/Select spread.”

Ultimately, we may see an industry where animals are sold at feeder calf sales and priced for their DNA genetic potential, he adds. Bulls and replacement females could be marketed that way, too.

“All of this is still a few years off,” Fluharty says. But, he adds, “GeneSTAR is a positive step, but it's just the start of what we're going to see. However, remember that selection is more complex than just marbling and tenderness.”

For now, a good rule appears to be: DNA marker-assisted selection should be just that — assistance. Genetics, nutrition, health programs and marketing will still be essential to produce high-quality beef.

Pollak sums it up: “Individual DNA tests are not the silver bullets. However, the more genes that are validated, the more capable we become in starting to build genotypes that put odds in favor of an animal carrying beneficial alleles.

“A DNA test (GeneSTAR) is going to be available in the U.S. In the short term, animals can be tested, and selection for marbling could be based on that. But, in the long term, we hope to incorporate that information into an EPD, and then producers can base selection on that.”

Soon producers should be able to select from beef sires that have EPDs for carcass traits and have been tested to determine if they carry gene markers for specific carcass traits.

Those results are expected after an evaluation of over 11,000 carcasses sired by more than 300 bulls from 15 breeds. The research is part of the National Carcass Merit Project initiated two years ago and funded through beef checkoff dollars.

The ultimate goal of the national project is to collect carcass data that individual breed associations can use to calculate EPDs for carcass traits like tenderness, marbling and ribeye area, according to Dan Moser. A second aspect of the project is to validate eleven gene markers identified in previous checkoff-funded research. This will help determine the effect of those markers on carcass traits in those breeds, Moser says.

Moser is an assistant professor of animal breeding and genetics at Kansas State University and serves as Breed Association Liaison for the Carcass Merit Project.

While the project won't be completed until the fall of 2002, some of the information is already available to breed associations, according to Moser. The American Simmental Association has already used the data to formulate tenderness EPDs on a few Simmental sires.

Of the marker tests he says, “It looks promising. We have seen significant marker effects on a majority of the bulls analyzed.” This data will also go to breed associations to disseminate.

While the aim of this project is to identify which sires carry which markers for certain carcass traits, Moser thinks the best way to use the marker information will be to add it to the calculations for specific EPDs.

“By incorporating genetic marker information into an EPD, it's a more acceptable format to be used by producers,” Moser says. The added information should also make those EPDs more accurate, he says.

In the future, a marbling test like GeneSTAR could be used to collect data to calculate EPDs, especially on young sires who don't have actual data from a lot of progeny, he says.